Cascode amplifiers typically include a transconductance amplifier coupled to a current buffer. Existing cascode amplifiers may include a pair of field-effect transistors coupled together, with one converting an input signal voltage to a signal current and having a common source and the other acting as current buffer and having a common gate. When the cascode amplifier is used in a switched-capacitor configuration, the switched-capacitor circuit may sample in a first period of the clock cycle and the amplifier may amplify the sampled signal in a second settling period of the clock cycle. The length of this settling period depends on the clock of the switches. A transient surge may occur at the amplifier input at the beginning of the settling period. As the settling period progresses, feedback at the amplifier input may cause the amplifier input signal to settle towards a final value, which may be reached at the end of the settling period. The amplifier output also reaches its final value at the end of the settling period.
In an NMOS transistor cascode amplifier, a transient surge of the amplifier input voltage causes the current flowing through the common-source input transistor and common-gate buffer transistor to increase. The voltage at the source of the common-gate transistor also decreases, since a fixed bias voltage is applied to its gate. If this voltage drop is large enough, the common-source input transistor leaves the saturated region and enters the triode region, which in turn limits the amplifier slew-rate.
As the settling time is fixed, the limited slew rate decreases the settling accuracy of the amplifier. While increasing the amplifier bandwidth in these instances had been shown to offset the decreased settling accuracy, the increased amplifier bandwidth also increased the resulting noise. Thus, a tradeoff existed between settling accuracy and noise in cascode amplifiers. The inventors perceive a need to increase the slew rate of cascode amplifiers in order to improve the settling accuracy, reduce the small signal bandwidth of the amplifier, and/or reduce the accompanying noise.